This invention relates to silicone rubber compositions. More particularly, this invention relates to a silicone rubber composition that prior to curing has excellent discharge and flow characteristics and that cures into a highly transparent silicone rubber.
The use of low-molecular-weight organosilicon compounds to improve the storage stability of silicone rubber compositions is already known. These organosilicon compounds are generally known as "plasticizers". As an example of these plasticizers, U.S. Pat. No. 2,890,188 teaches the use of diorganooligosiloxane endblocked at both terminals by silanol. Among plasticizers of this type, .alpha.,.omega.-dimethyloligosiloxanediols are widely used as plasticizers for silicone rubber compositions. In addition, Japanese Patent Publication (Kokoku) Number Hei 4-22179 (22,179/1991) teaches the use of hexaorganodisilazane as plasticizer while Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 5-140457 (140,457/1993) teaches plasticizer in the form of a diphenylsiloxane unit-containing organooligosiloxane endblocked by silanol at a single terminal.
However, silicone rubber compositions containing .alpha.,.omega.-dimethyloligosiloxanediol plasticizers suffer from several problems: (i) they have poor discharge or flow characteristics prior to their cure, and (ii) they cure into silicone rubbers with an impaired transparence. The use of a silazane plasticizer, although not causing a decline in the transparency of the silicone rubber, does cause the silicone rubber to yellow during molding. Finally, a poor transparency is exhibited by silicone rubber compositions that contain plasticizer in the form or diphenylsiloxane unit-containing oligosiloxane endblocked by silanol at a single terminal.
The object of the present invention is to provide a silicone rubber composition that has excellent discharge characteristics and flow characteristics prior to its cure and that cures into a highly transparent silicone rubber.